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Recommendation: Boron Nitride Hardcoat®, MW-50 Paste
Boron Nitride LUBRICOAT® or LUBRICOAT-BLUE® is recommended for this use because of its superior Teflon®-like release properties. For auto-pour ladles, this is the property of most importance. If aluminum skin builds up as it does with traditional coatings, mispours can occur because the shot size is reduced and/or the lip area is obstructed.
The primary problem to be solved by a coating is to seal porosity that exists in ceramic staulk tubes. Both of the above products have been demonstrated to work and the coating selection is primarily driven by shop practices.
Boron Nitride HARDCOAT® is applied to both the inside and outside of staulk tubes using either standard brushing or dipping methods. If brushed, a roller or sponge attached to a pole facilitates coating the inside surfaces. For dipping, a PVC pipe capped at one end and filled with Boron Nitride HARDCOAT® is one method used. If dipped, as much of the excess paint needs to be removed since thick coatings are much more likely to flake in service.
MW-50 should only be applied by brushing to the outside of the staulk tubes. Unlike Boron Nitride HARDCOAT®, a thick (about 10 mils or more) coating is recommended (fiber reinforcement used in MW-50 permits thick coatings). Of course, these thick coatings require longer time to thoroughly dry; and if not thoroughly dry before use, the coating will flake during use. Many users employ a drying oven.
Performance data suggests that the MW-50 is best at sealing the porosity; but, of course, skin and dross will adhere unlike what would be observed with the Boron Nitride HARDCOAT®. Some users will top the MW-50 with BN LUBRICOAT® . Why LUBRICOAT® and not HARDCOAT®? MW-50 uses an acidic binder- same as LUBRICOAT®. HARDCOAT® uses a basic binder. If these two binders are not the same, an acid-base reaction can occur causing coating flaking. Combo Coatings
are sometimes helpful in achieving optimum performance ... but always require consideration of acid-base coating reactions.
Coating Maintenance Guide
Generally no coating maintenance is used.
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